Method of conserving pickling liquor



Patented ay 31,1938

NITED STATES on or coNsERvme m i r LIQUOR Hans Barkholt, Berlin-Halensee, Germany, as-

signor to & 00., G. m. b. E, Berlin, ottermany No Drawing. Application January 30, 1936,

Serial No. 61,547

furic acid content of the bath is normally gradually depleted andthe operation of the bath is also slowed up by the formation of ferrous sulfate (FeSO4 plus 7H2O), the gradual accumulation of which renders the bath-viscous and sluggish.

' In the earlier days of sulfuric acid pickling and to some extent at the present time, the original pickling bath is used until-the strength of the 29 acid becomes so depleted that it no longer satisfactorily removes the scale, whereupon the bath is discharged to waste, sometimes contaminating nearby streams and offering other objections to the community, and a new bath is prepared. This practice-leads not only to a waste of the sulfuric-acid still contained in the bath and the ferrous sulfate, but also in the event the ferrous sulfate and sulfuric acid liquors reach open streams, to the destruction of animal life in the streams, etc.

More recently various processes have been suggested and/or used for the recovery of the sulfuric acid and the ferrous sulfate and to prevent pollution of streams, etc., as, for example, the processes described in the Marsh and Cochran patents, Reissue No. 15,119, granted June 7, 1921, No. 1,450,216, granted April 3, 1923, No. 1,392,780, granted Oct. 4, 1921, and various other patents and literature publications. In all of these prior 40 patents and processes for the recovery of pickling liquor and ferrous sulfate, it has been suggested to use the pickle liquor to the point where the sulfuric acid content has become depleted or the content of ferrous sulfate has slowed up the operation of the bath until it is no longer economical, and then to remove the pickling liquor. from the vat and separate the ferrous sulfate from the sulfuric acid pickling liquor, either by evaporating and concentrating the spent liquor to a point where the ferrous sulfate crystals could be economically removed or by the addition of fresh acid to bring the relative concentration of sulfuric acid to the ferrous sulfate to a point where the major portion of the ferrous sulfate could be removed by chilling and centrifuging or, by other mechanical methods of separation. Under these methods the period of time in which a pickling bath can be efficiently used is of limited duration .with the necessity for frequent removals of the pickling liquor from the pickling vats for the purpose of separating ferrous sulfate therefrom.

It is the object of my invention to provide a method whereby the operation of the original pickling liquor in the pickling vat can be eficiently continued over a long period of time and until the concentration of ferrous sulfate in the pickling vat becomes high enough to permit its more economical removal in large quantities, and

then to remove the pickling liquor from the pickling vat for the purpose of separating the ferrous sulfate from the pickle liquor so that the pickle liquor is in process of recovery and regeneration for only a relatively short period of time as compared with the period of time with which pickling liquor is actuallyused in the process of pickling.

I have discovered that sufficient ferrous sulfate may be removed from a pickling bath containing a relatively high concentration of sulfuric acid to permit efiective regeneration of the bath by chilling and centrifugal separation of the precipitated ferrous sulfate crystals without concentration of the bath by heat or the like, and that newsupplies of concentrated sulfuric acid may be added to the bath during the pickling operation to 1 maintainits relative pickling strength even in the face of increasing concentrations of ferrous sulfate without introducing complications into the process, as long as the concentration of sulfuric acid and the temperature of the bath do I wire works, the strength of the pickling bath will be approximately 7.5% by weight of sulfuric acid or between 5 and 10%. In sheet and tube mills, the pickling bath will contain from 10 to 15% or preferably around 12 to 13% of free sulfuric acid by weight. The experience of the particular mill, the degree of scale of the objects to be pickled and other factors will control the relative starting concentration of the bath.

During the pickling operation, the strength of the acid is maintained between desired limits as, for example, between 5 and 10% in a wire mill, or 10 to 15% in a sheet and tube mill, by frequent additions of concentrated acid during the pickling operation. It is possible to determine the relative content of sulfuric acid in the bath by methods of chemical test in the control laboratory. In practical operation, however, the concentration of sulfuric acid may be determined by practical tests, such as the ebullitlon of hydrogen at the surface of the bath and the period of time which is taken to effect satisfactory pickling of the particular materials. An experienced pickling operator can readily tell from these practical indications when the strength of the acid is too great or too weak and can thereby in a practical way control the rate of addition of concentrated acid to the bath as satisfactorily as by chemica testing methods.

During the pickling operation'increased sluggishness or viscosity imparted to the bath by the accumulation of ferrous sulfate therein may be overcome by gradually increasing the temperature of the bath. In this way and by frequent additions of new concentrated sulfuric acid to the bath, the relative strength of the bath and the duration of the pickling operation may be kept substantially constant, while the gradual accumulation of ferrous sulfate in the bath, which normally would increase the viscosity of the bath and slow down the pickling operation, may be overcome by gradually increasing the temperature of the bath, the pickling vats usually being provided with steam coils or direct steam addition to enable them to be heated.

When the pickling operation has been confrom the mother liquor by centrifuge operation or in another suitable manner, and the mother liquor returned to the cycle for use as afresh pickling bath in the manner previously described.

In order to guide persons skilled in the art in practicing my invention, the following tables are given to show the effect of increasing the temperature of the bath in overcoming the viscosity imparted by'the accumulation of ferrous sulfate and the effect of increasing the sulfuric acid concentration in a bath at a constant temperature and how both the temperature and the sulfuricacid concentration may be increased to maintain the relative strength of the bath at a substantially constant duration during the pickling operation.

TABLE I For constant acid content and increase in temperature of the bath according to increase in concentration of ferrous sulfate:

Duration g FebO; Temperg H SO /l plus 7 ature of of figg HjO/l the bath minutes C. 110 0 40 9% 110 100. 40 10 110 200 50 10 110 300 60 10 110 400 70 10% I10 500 l0 600 80 12% TABLE II For increase in acid concentration with constant For simultaneous increase in temperature of the bath and simultaneous increase in concentration of acid as content of ferrous sulfate increases:

Duration. g FeSO; Temperag u,so. 1 plus ture of g g f 7 1110 1 the bath minutes In the above tables th? concentration of sulfuric acid and of ferrous sulfate is given in terms of grams per liter. From these tables it is apparent that in order to maintain uniform pickling duration in the face of increasing concentrations of ferrous sulfate, it is necessary either to maintain the sulfuric acid content substantially constant by frequent additions of acid or'to increase the content of sulfuric acid and the temperature of the bath simultaneously.

In order to remove the ferrous sulfate from a solution of pickling liquor having for example 600 g. of FeSOa plus 7H2O/1,- if the liquor for example contains g. of H2804 per liter, the solution is carried down to 0 C., whereupon 485 g. of. the ferrous sulfate will separate in crystalline form. from each liter of the waste bath which constitutes about 80% of the total quantity of ferrous sulfate, whereas approximately 115 g. will remain dissolved. The crystallized ferrous .sulfate may be removed by centrifuge, filtration or other suitable operations, and the mother liquor returned to the pickling vat. Any deficiency in volume may be'replaced by using the Wash waters from the pickling vat, which wash anacoa waters under normal conditions usually contain from 10 to 30 g. of free H2304 per liter. The resulting pickling liquor may then be re-used in the manner previously described by further additions of concentrated sulfuric acid as the strength of the liquor becomes depleted and the ferrous sulfate content increases until the amount of ferrous sulfate has again reached 32 to 42% of. the total, whereupon the pickling liquor may be again withdrawnand the ferrous sulfate removed and the pickling liquor again returned to the pickling vats. In this manner the strength of the pickling liquor and theduration of the pickling operation are maintained substantially constant, no pickling liquor is wasted and the recovered ferrous sulfate may be sold in commerce for use in the manufacture of pigments, water purification, etc., the value of the ferrous sulfate recovered being more than the cost of operation of the recovery process in addition to which none of the more valuable pickling liquor is wasted.

While as stated above my invention may be applied in diiferent ways according to the needs of the various mills and the only limiting factor on the continued addition of fresh acid and the continued use of the pickling vat is the necessity for avoiding acid concentrations which will dehydrate the ferrous sulfate crystals, the following specific examples illustrate how the invention may be applied in practice.

Example I.--In a wire mill, a fresh pickling bath is made with 7.5% of sulfuric acid by weight. At certain intervals during the wire pickling operation, new concentrated acid is added to the vat so that the acid content remains within the limits of to and the bath is gradually heated to overcome the increased viscosity imparted thereto bythe accumulation of ferrous sulfate. In order to avoid the formation of low hydrated ferrous sulfate (F8804 plus ll-I20 or F6804 plus 1H2O) care is taken that the ferrous sulfate (FeSOr plus 'IHz'O) does not accumulate to a greater concentration than 56% in a bath containing 5% H2504 by weight and that the temperature does not exceed 68 C. a

For a bath containing 7.5% free of H2504, the copperas (FeSO4 plus II-I20) should not be permitted to accumulate beyond 52.5% concentra tion and the temperature of the bath should not -rise over 71 C. Within these limits the pickling operation is continued with frequent additions of concentrated sulfuric acid and increase in temperature of the bath until the liquor normally contains 35 to 45% of copperas, whereupon the liquor is removed and cooled to 0 to 20 C. according to the season, and the copperas crystals thus formed are separated from the mother liquor by centrifuge or other operation and the mother liquor returned to the cycle for fresh use in the pickling bath. I

Example II .In a sheet and tube mill, a fresh pickling bath is prepared containing from 10 to of free sulfuric acid by weight and the bath is used in such manner that concentrated sulfuric acid is added so that the acid content is kept substantially constant and does not exceed, for example, more than or less than 8% in weight. The more nearly uniform the concentration is kept the more constant the pickling operation. This bath is operated at a gradual increase in temperature as the ferrous sulfate.

content increases until in operation 32 to 42% of copperas is in the bath. The precipitation of lower hydrates of ferrous sulfate is avoided by not permitting the bath to exceed the following I limits:

For 8% free acid, 51% copperas and 73 C.

For 12.5% free acid, 45% copperas and 73 C.

When the content of copperas approaches these limits or is between the limits of 32 to 42%, the pickling liquor is withdrawn from the vat and cooled to a temperature of 0 to 20 C. and the precipitated ferrous sulfate crystals separated as previously described and the mother liquor is returned to the pickling operation.

While I have set forth these two specific examples, it will be understood that persons skilled in the art may operate the pickling process satisfactorily within the limits of concentrations and temperatures above given to maintain a substantially uniform duration of pickling operation and to permit a high concentration of ferrous sulfate in the pickling liquor before removing the pickling liquor from the pickling operation for the purpos of separating ferrous sulfate crystals.

It is possible to apply this process, depending upon the products to be pickled, within the limits of 3 to 40% of free acid and also to remove the pickling liquor and precipitate the copperas before it has reached concentrations above for example in concentrations between 10 and 30%. Such operation, however, necessitates frequent removal of the pickling liquor from the pickling vat and is not as economical as the process described wherein the copperas contained in the pickling liquor is permitted to. accumulate to between 32 to 45%.

Various modifications and changes may be made from the specific examples given without departing from the spirit of my invention and the scope of the appended claims.

I claim:

1; The method of performing successive pickling operations on iron and steel articles in a bath and conserving sulfuric acid pickling liquor used therein, which comprises employing the bath initially while at a relatively low temperature, maintaining the strength of the liquor by .frequent additions of sulfuric acid during the pickling operations, using the pickling liquor until the content of ferrous sulfate has risen to 30 to 45%, meanwhile gradually increasing the temperature of the bath as the ferrous sulfate conwhich comprises employing said bath initially while at a low temperature within the range of about 40 to 80 C. and at an acid concentration between about 10 and 18%, maintaining the strength of the liquor by frequent additions of sulfuric acid during the pickling operations, gradually increasing the temperature of the bath to a high temperature within the said range as the ferrous sulfate content increases, and using the pickling liquor only until the content of ferrous sulfate has risen to 30 to percent removing ferrous sulfate by chilling the liquor and separating the resulting ferrous sulfate crystals therefrom, and reusing the purified liquor in the same cycle of operation.

3. The method of performing successive pickling operations on iron and steel articles wherein such articles are immersed successively into a sulfuric acid bath which comprises employing said bath initially while at a low temperature within the range of about to 80 C., maintaining the strength of the liquor by frequent additions of sulfuric acid during the pickling operations, using the pickling liquor until the content of ferrous sulfate has risen to 30 to percent, meanwhile gradually increasing the temperature of the bath to a high temperature within the said range as the ferrous sulfate content increases whereby the time required for successive pickling operations is substantially constant, removing ferrous sulfate by chilling the liquor and separating the resulting ferrous sulfate crystals therefrom and reusing the purified liquor in the same cycle of operation.

4. The method of performing successive pickling operations on iron and steel articles in a sulfuric acid bath which comprises employing said bath initially at a low temperature within the range of 40 to C., maintaining the strength of the liquor by frequent additions of sulfuric acid during the pickling operations, meanwhile gradually increasing the temperature of the bath during said operations to a temperature substantially up to 73 to 80 C.' as the ferrous sulfate content increases whereby the time required for successive pickling operations is substantially constant, discontinuing the use of said liquor-when the content of ferrous sulfate has risen to a point in excess of 30%, precipitating ferrous sulfate from the liquor by cooling, and then removing the precipitated ferrous sulfate and returning the purified pickling liquor for reuse according to the method aforesaid.

5. The method of performing successive pickling operations on iron and steel articles in a wire mill wherein such articles are immersed into a sulfuric acid bath which comprises employing the bath initially while at a low temperature within the range of about 40 to 80 C. and at an acid concentration between about 4 and 10%, maintaining the strength of the liquor by frequent additions of sulfuric acid during the pickling operations, using the pickling liquor until the content of' ferrous sulfate has risen to a point in excess of 30%, meanwhile gradually increasing the temperature of the bath to a high temperature within said range as the ferrous sulfate content increases whereby the time required for each pickling operation is substantially constant, removing ferrous suilfate from the liquor by chilling the same and separating the precipitated ferrous sulfate crystals therefrom and reusing the purified pickling liquor in the method aforesaid.

HANS BARKHOLT. 

